For any PDF, Arabic or any other language, I'd recommend a large screen e-Reader, either PB902/912 or Onyx Boox M92 - both of them do a great job of reflowing PDFs.
Though i believe that M92 is a little better device in general, but specifically for Arabic, I'd recommend PocketBook, as it is more suited for multilingual support and the actually do have Arabic menu.
If you have a PDF or two in Arabic, you can send it to me at
info@ereadersmarket.com, I'll put it on PocketBook and will send you the screenshot of how it is handling it.